In the prior art, three types of compensation have been proposed. Fixed compensation involved using a capillary or orifice to act as a fixed value resistance. Variable compensation included the use of diaphragm and valves to provide a flow inversely proportional to the pocket resistance, thereby creating a larger pressure differential than created with the use of a fixed compensation device. Both of these types of compensation, however, must be tuned to the bearing gap.
As smaller and smaller bearing gaps are sought in order to increase the performance of the bearing, manufacturing errors make the use of either of these types of compensation more and more difficult by requiring hand-tuning of the compensation device. Since a machine tool with three axes may have 36 bearing pockets, the labor required becomes prohibitive.
A third type of compensation is called self-compensation because it uses the change in bearing gap to allow the bearing to change the flow of fluid to the pockets, by itself. Existing self compensation methods have utilized linear passageways on the face of the bearing and have been directed primarily to applications in spindles, as later more fully discussed. These designs have not, however, proven themselves to provide acceptable performance in the commercial sector because of inefficient flow patterns that are difficult analytically to determine, particularly the flow field near the end of the linear grooves, often resulting in improper resistance design and which then require hand-tuning of the compensator. Difficulty has also been experienced with prior linear groove self-compensation units because the geometry has not always been realistically implementable.
Underlying the present invention, is the discovery that through the use of a self-compensating unit in the form of a pressurized annulus that feeds the fluid to a hole in its center which is then connected to a bearing pocket on the opposite side of the bearing, the limitations of such prior art approaches are admirably eliminated. The annulus, however, is easy to manufacture and is more structurally stable than linear passages; and, furthermore, the fluid flow from the circular annulus to the center feed hole can be analytically determined with great accuracy.